The 1977-1978 Eruption of Usu Volcano

The 1977-1978 activity of Usu Volcano began with a series of major eruptions of dacite pumice from August 7 to 14, 1977, which was preceded by a local earthquake swarm. After the First Stage eruptions, due to rising of the residual viscous magma, notable crustal movements accompanied by earthquakes continued and fumarolic activity increased at the summit. In November, 1977, the Second Stage eruptions broke out with a minor phreatic explosion and intermittently occurred in 1978 gradually increasing in scale of activity. Thus, moderate phreatomagmatic and magmatic eruptions as well as phreatic ones occurred in August and September, then the eruptive activity stopped in October. Subsequent to our first report on the First Stage eruptions (Katsui, et al., 1978), the whole sequence of the Second Stage eruptions is described here, together with the nature of the ejecta, and character and scale of the eruption.